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  • H01F41/00—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties
  • H01F41/02—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties for manufacturing cores, coils, or magnets
  • H01F41/0206—Manufacturing of magnetic cores by mechanical means
  • H01F41/0246—Manufacturing of magnetic circuits by moulding or by pressing powder
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  • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
  • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
  • H01F1/34—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials non-metallic substances, e.g. ferrites
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
  • C04B35/26—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on ferrites
  • C04B35/265—Compositions containing one or more ferrites of the group comprising manganese or zinc and one or more ferrites of the group comprising nickel, copper or cobalt
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  • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
  • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
  • H01F1/34—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials non-metallic substances, e.g. ferrites
  • H01F1/342—Oxides
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  • H01F17/00—Fixed inductances of the signal type 
  • H01F17/04—Fixed inductances of the signal type  with magnetic core

Definitions

• This invention relates to a ferrite core which is adapted for use in a transformer or a choke coil used at a high temperature, and its production method.
• this invention relates to a ferrite core which exhibits a high saturation flux density at a high temperature of 100° C. or higher, and in particular, at a temperature around 150° C., and which has high magnetic stability with reduced deterioration in high temperature storage, as well as its production method.
• a soft ferrite which is used in producing a magnetic core should have a high saturation flux density and a low power loss.
• Such ferrite can be used as a ferrite core in a transformer or a choke coil of a DC—DC converter in an EV (electric vehicle) or HEV (hybrid electric vehicle), or as a ferrite core to be placed near the engine of an automobile which will be exposed to a high temperature.
• Such soft ferrite core which is used at a high temperature.
• exemplary such properties include excellent durability with reduced magnetic deterioration during use at a high temperature, the saturation flux density which experience no significant decrease at a high temperature, and low power loss.
• JP-A 10-64715 proposes a magnetic core material of low loss ferrite comprising a MnZnNi ferrite in order to provide a ferrite magnetic core material which exhibits a low loss and a high saturation flux density for a relatively broad frequency band of about 100 kHz to 500 kHz.
• the magnetic core material of MnZnNi ferrite disclosed in JP-A 10-64715 was still insufficient in the saturation flux density Bs and loss at a high temperature of 100° C. or higher, and in particular, at around 150° C. as well as in the magnetic stability although it had sufficiently high saturation flux density Bs and sufficiently low loss at 80° C.
• JP-A 2-83218 also proposes an oxide magnetic material of MnZnNi ferrite. This material has been developed to provide a material which has highly stable magnetic properties, high saturation flux density, and low power loss when used at a high temperature range of 100° C. or higher, and in particular, at 100 to 200° C. at magnetic field strength (flux density) of 1000 G (100 mT) or higher, and in particular, at 2000 to 5000 G (200 to 500 mT) or higher.
• flux density magnetic field strength
• JP-A 2-83218 additives incorporated as auxiliary components are particularly defined.
• the material disclosed in JP-A 2-83218 exhibits dramatically improved saturation flux density in view of the state of the art at that time.
• An object of the present invention is to obviate the situation as described above, and to provide a ferrite core which has high saturation flux density Bs at a high temperature of 100° C. or higher, and in particular, at around 150° C., and which has excellent magnetic stability at a high temperature, experiencing reduced deterioration of magnetic properties, and in particular, reduced core loss at such high temperature (even by trading off some improvement in the level of the loss).
• a ferrite core according to the above (1) containing 56 to 57 mol % of iron oxide calculated in terms of Fe 2 O 3 , 5 to 10 mol % of zinc oxide calculated in terms of ZnO, 3 to 6 mol % of nickel oxide calculated in terms of NiO, and the balance of manganese oxide (MnO) as its main components.
• a core loss at 100° C. of up to 1200 kW/m 3 when measured by applying a sine-wave AC magnetic field of 100 kHz and 200 mT.
• a core loss at 100° C. of up to 900 kW/m 3 when measured by applying a sine-wave AC magnetic field of 100 kHz and 200 mT.
• the firing step comprises heating stage, steady temperature stage, and cooling stage in this order, and
• the article is kept in the steady temperature stage at a temperature (steady temperature) of at least 1250° C. with the oxygen concentration of the atmosphere kept at 0.05 to 2.0%.
• Tn a specific temperature in 900 to 1200° C.
• a is 3 to 14, and b is 5000 to 23000, provided that a and b may or may not alter with the decrease in the temperature T;
• the temperature is reduced from Tn to the room temperature at a cooling rate which is 2 to 10 times faster than the cooling rate used in the cooling from the steady temperature to the temperature Tn.
• the substantial component of the ferrite core of the present invention is constituted by the main components comprising 55 to 59 mol %, and preferably 56 to 57 mol % of iron oxide calculated in terms of Fe 2 O 3 , more than 0 to 15 mol %, and preferably 5 to 10mol % of zinc oxide calculated in terms of ZnO, 2 to 10 mol %, and preferably 3 to 6 mol % of nickel oxide calculated in terms of NiO, and the balance of manganese oxide (MnO).
• MnO manganese oxide
• the ferrite core of the present invention may further comprise various known auxiliary components in addition to the main components as described above.
• SiO 2 0.005 to 0.03 mass %
• V 2 O 5 0.01 to 0.1 mass %
• auxiliary components may be incorporated either alone or in combination of two or more.
• SiO 2 and CaO are the most preferred.
• the content of SiO 2 is less than 0.005 mass %, or the content of the CaO is less than 0.008 mass %, the resulting ferrite will suffer from reduced electric resistance, and hence, increased power loss.
• the SiO 2 content is in excess of 0.03 mass %, or the CaO content is in excess of 0.17 mass %, abnormal grain growth will take place in the firing, and it will be difficult to obtain the desired saturation flux density Bs and the low power loss.
• the value of ⁇ (amount of excessive oxygen or cation vacancies) in the formula (1) is such that ⁇ 2.5 ⁇ 10 ⁇ 3 , preferably ⁇ 2.0 ⁇ 10 ⁇ 3 , more preferably, ⁇ 1.0 ⁇ 10 ⁇ 3 , and most preferably ⁇ 0.5 ⁇ 10 ⁇ 3 .
• the value of ⁇ When the value of ⁇ is too large, it is highly likely that stability of the magnetic properties at high temperatures becomes insufficient, and in particular, increase in the core loss and decrease initial permeability ⁇ i at a temperature higher than the secondary peak temperature of the ferrite become significant. It is to be noted that the value of ⁇ may be equal to zero. However, when the firing conditions are controlled such that the ⁇ value is zero, it will then be difficult to realize the desired magnetic properties in a stable manner, and the ⁇ value is preferably larger than zero.
• ⁇ is calculated from the results of analysis of the composition and quantitative analysis of Fe 2+ and Mn 3+ .
• the composition was analyzed by pulverizing MnZnNi ferrite sintered body, and evaluating the MnZnNi ferrite powder with an X-ray fluorescence analyzer (for example, Simultix 3530 manufactured by Rigaku) by glass bead method.
• an X-ray fluorescence analyzer for example, Simultix 3530 manufactured by Rigaku
• the Fe 2+ and Mn 3+ were quantitatively analyzed by pulverizing the MnZnNi ferrite sintered body, dissolving the resulting powder in an acid, and thereafter conducting potentiometric titration using K 2 Cr 2 O 7 solution.
• the content was calculated by assuming that all of the Ni and Zn found in the analysis of the composition were present as divalent ions.
• the amounts of the Fe 3+ and Mn 2+ were assumed to be the values obtained by subtracting the amounts of Fe 2+ and Mn 3+ determined by the potentiometric titration from the amounts of the Fe and Mn determined in the analysis of the composition.
• the ferrite core of the present invention is produced by firing the article molded from the powder of starting materials as in the case of conventional ferrite cores.
• the powder of starting materials may be produced either by calcining the starting materials, or by directly roasting the starting materials with no calcination step.
• composition of the main component is preferably limited to the composition as described above.
• the firing step is preferably accomplished by heating stage, steady temperature stage, and cooling stage which are conducted in this order.
• the oxygen concentration of the atmosphere is preferably controlled to 10% or less, and more preferably to 3% or less, and the heating rate is preferably controlled to 50 to 300° C./hr, and more preferably to 50 to 150° C./hr.
• the control of heating conditions in the heating stage does not significantly affect to the control of the ⁇ value.
• the control of the heating conditions results in the production of a compact ferrite core, and hence, in an improved saturation flux density with a reduced core loss.
• the oxygen concentration may exceed the range as specified above, and may be equivalent to the oxygen concentration in the air.
• the temperature is maintained at an adequately selected steady temperature of about 1250 to 1400° C.
• the firing atmosphere used is a relatively oxygen-poor atmosphere which has never been employed in the art, and to be more specific, the firing atmosphere has an oxygen concentration of 0.05 to 2.0%, and preferably 0.05 to 0.8%.
• the cooling stage is accomplished such that, when a specific temperature in 900 to 1200° C. is designated Tn, and the temperature is reduced from the steady temperature to the temperature Tn, the oxygen concentration of the atmosphere P O 2 (unit: %) at temperature T (unit: K) is either gradually or incrementally reduced to satisfy the relation:
• a is preferably 3 to 14, more preferably 5 to 13, and most preferably 7 to 11; and b is preferably 5000 to 23000, more preferably 8000 to 21000, and most preferably 11000 to 19000.
• a and b When the oxygen concentration PO 2 is continuously reduced with the decrease in the temperature T, a and b may be typically set at a particular value, respectively.
• a and/or b when the oxygen concentration PO 2 is incrementally reduced with the decrease in the temperature T, a and/or b may be altered, in the temperature range wherein the PO 2 is to be maintained at the constant value, so that a ⁇ b/T remains at a constant value.
• a and b may be either altered in accordance with the decrease in the temperature T, or kept at constant values irrespective of the decrease in the temperature T.
• the temperature range wherein the oxygen concentration is to be maintained at the constant value preferably does not exceed 100° C.
• the merit of reducing the oxygen concentration with the decrease in the temperature will be less significant.
• the specific values for a and b may adequately determined to thereby obtain the best results.
• the temperature is reduced from the steady temperature to the temperature Tn preferably at a cooling rate of 20 to 200° C./hr, and in particular, at 40 to 150° C./hr.
• the temperature is reduced from Tn to the room temperature at a cooling rate which is 2 to 10 times faster than the cooling rate used in the cooling from the steady temperature to the temperature Tn.
• the decrease in the oxygen concentration from the steady temperature to the temperature Tn may be accomplished by reducing the ratio of the oxygen gas or the air mixed in the gas other than the oxygen (nitrogen gas, inert gas, or the like), and the ratio of the oxygen gas or the air mixed is typically reduced to zero at temperature Tn.
• the oxygen concentration will not be reduced exactly to zero due to the inevitably remaining or generating oxygen gas even when the ratio of the oxygen gas or air the mixed were reduced to zero.
• the ⁇ value will not be significantly affected by the oxygen remaining at the concentration as low as about 0.01% at the temperature lower than the temperature Tn due to the increased cooling rate in such temperature range.
• the temperature Tn may be adequately determined to thereby obtain the best results.
• the gas constituting the atmosphere other than the oxygen substantially comprises nitrogen or an inert gas.
• the firing temperature (steady temperature) used may be at least 1250° C., preferably up to 1400° C., and more preferably 1300 to 1360° C., and the oxygen concentration used in the steady temperature stage in the firing is as described above.
• the firing temperature is less than 1250° C., sintering density will be unduly low, and as a consequence, the product will suffer from a low saturation flux density and an increased core loss.
• an excessively high firing temperature is likely to invite abnormal grain growth and an increased core loss.
• the oxygen concentration in the steady temperature stage in the firing is too high, increase in the core loss during the high temperature storage will be increased.
• the oxygen concentration in the steady temperature stage may be reduced to 0% in view of suppressing the increase of the core loss, it will then be difficult to obtain the desired electromagnetic properties at such an extremely low oxygen concentration in the steady temperature stage, and the core loss will be particularly increased. Therefore, the oxygen concentration is preferably not reduced beyond the range as specified above.
• the firing time (the time of the steady temperature stage) used may be substantially the same as the one used in the conventional ferrite production process, and most typically 2 to 10 hours.
• the conditions employed in the steps of calcination, roasting, molding, and the like may also be similar to those employed in the conventional ferrite production process.
• the pressure used in the molding may be 48 to 196 MPa.
• the ferrite core of the present invention exhibits excellent magnetic properties at a high temperature.
• the ferrite core of the present invention exhibits a saturation flux density at 100° C. of 430 mT or more, or 450 mT or more, or even 455 mT or more, a saturation flux density at 150° C. of 350 mT or more, or 380 mT or more, or even 385 mT or more when measured by applying a magnetic field of 1000 A/m, and a core loss at 100° C. of 1200 kW/m 3 or less, or 900 kW/m 3 or less, or even 750 kW/m 3 or less when measured at 100 kHz, 200 mT.
• the ferrite core of the present invention also exhibits an increase in the core loss of up to 4%, or up to 3%, or even up to 1% when stored at 150° C. for 2000 hours, an increase in the core loss of up to 10%, or even up to 5% when stored at 175° C. for 2000 hours, and an increase in the core loss of up to 50%, or up to 40%, or even up to 30% when stored 200° C. for 2000 hours.
• the ferrite core samples shown in Table 1 were produced by the procedure as described below.
• the materials for the main components were prepared to comply with the composition shown in the Table 1, wet mixed, and after drying with a spray dryer, calcined at 900° C. for 2 hours.
• the materials for the main components were prepared to comply with the composition shown in the Table 1, wet mixed, and after drying with a spray dryer, calcined at 900° C. for 2 hours.
• the mixing was accomplished by adding the auxiliary component materials to the calcined main component materials, and pulverizing the mixture until mean particle diameter of the calcined materials was 1.5 ⁇ m.
• PVA polyvinyl alcohol
• the powder was molded by applying a pressure of 98 MPa (1 ton/cm 2 ) to thereby produce a toroidal magnetic core sample.
• the resulting toroidal magnetic core sample was fired under the conditions as specified below. It is to be noted that, in the heating stage, steady temperature stage, and cooling stage, the gas other than the oxygen constituting the atmosphere was nitrogen.
• the temperature was raised from room temperature (R.T.) to 900° C. at a rate of 300° C./hr, and from 900° C. to the steady temperature at a rate of 100° C./hr.
• the oxygen concentration in the atmosphere was controlled so that the oxygen concentration was up to 3% at the temperature of 600° C. or higher.
• the temperature was kept at the steady temperature of 1300° C. for 5 hours.
• the oxygen concentration in the steady temperature stage was selected from the range of 0.05 to 2.0%.
• the temperature was reduced from the steady temperature to 1000° C. at a rate (cooling rate) of 50° C./hr, with the oxygen concentration controlled such that the oxygen concentration P O 2 (unit: %) at temperature T (unit: K) meets
• a is a particular value in the range of 7 to 11
• b is a particular value in the range of 11000 to 19000.
• the temperature was reduced from 1000° C. at a rate of 300° C./hr with the oxygen concentration of the atmosphere maintained at 0.01% or less.
• the samples were measured for the value of the core loss (power loss) Pcv, increase in the Pcv during storage at a high temperature, saturation flux densities Bs 100 and Bs 150 at 100° C. and 150° C., and the value of ⁇ by the procedure as described below.
• Core loss at 100° C. was measured with a B-H analyzer by applying a sine-wave AC magnetic field at 100 kHz and 200 mT (maximum value).
• the samples were stored in an atmosphere of 150° C. for 2000 hours, and measured for the core loss before and after the storage.
• the core loss before the storage was designated P cvb
• the core loss after the storage was designated P cva .
• Increase (%) in the core loss was calculated by the equation:
• Saturation flux densities Bs 100 and Bs 150 at 100° C. and 150° C. were measured with a B-H tracer, respectively, by applying a magnetic field of 1000 A/m.
• ⁇ was calculated from the results of analysis of the composition and quantitative analysis of Fe 2+ and Mn 3+ .
• the composition was analyzed by pulverizing MnZnNi ferrite sintered body, and evaluating the MnZnNi ferrite powder with an X-ray fluorescence analyzer (Simultix 3530 manufactured by Rigaku) by glass bead method.
• the Fe 2+ and Mn 3+ were quantitatively analyzed by pulverizing the MnZnNi sintered body, dissolving the resulting powder in an acid, and thereafter conducting potentiometric titration using K 2 Cr 2 O 7 solution.
• Ni 2+ and Zn 2+ it was assumed that all of the Ni and Zn found in the analysis of the composition were present as divalent ions.
• the amounts of the Fe 3+ and Mn 2+ were assumed to be the values obtained by subtracting the amounts of Fe 2+ and Mn 3+ determined by the potentiometric titration from the amounts of the Fe and Mn determined in the analysis of the composition.
• the Examples of the present invention exhibits high saturation flux density at 100° C. and 150° C. as well as reduced core loss, and furthermore, reduced increase in the core loss during the high temperature storage.
• the ferrite core samples of the present invention exhibited a core loss P CV of 1200 kW/m 3 or less, increase in the core loss P cv of 4.0% or less, a Bs 100 of 430 mT or more, and a Bs 150 of 350 mT or higher.
• initial permeability ⁇ i was measured before and after the high temperature storage to calculate the decrease in the initial permeability ⁇ i
• the samples of the present invention were also found to exhibit smaller decrease in the initial permeability ⁇ i.
• Sample No. 120 (Comparative Example) of Table 1 has the same composition of the main components as Sample No. 106 (Example of the present invention).
• the value of ⁇ is outside the scope of the present invention in the case of the Sample No. 120 since the oxygen concentration employed in the steady temperature stage was 6%.
• the ferrite core samples shown in Table 2 were produced by repeating the procedure of Example 1 except that the steady temperature in the firing and the oxygen concentration in the steady temperature stage used were as shown in Table 2.
• the resulting samples were evaluated for their properties as in the case of Example 1 except that the core loss Pcv were measured by storing at 150° C. for 2000 hours, 175° C. for 2000 hours, and 200° C. for 2000 hours.
• the ferrite core of the present invention has high saturation flux density at 100° C. and 150° C. as well as reduced core loss, and experiences reduced increase in the core loss in the high temperature storage. Therefore, the ferrite core of the invention has the properties required for the ferrite core used in a transformer or a choke coil of a DC—DC converter in an EV (electric vehicle) or HEV (hybrid electric vehicle) which will be used at a high temperature, or a ferrite core to be placed near an automobile engine which will also be exposed to a high temperature.
• EV electric vehicle
• HEV hybrid electric vehicle
• the improvement of the magnetic properties in the high temperature region in particular, the suppression of the increase of the core loss in the storage at a high temperature of 150° C. or more, it is believed that such improvement may be ascribed at least partly to the suppression of the value of ⁇ (cation vacancies) to the level below a certain value by limiting the composition of the ferrite as well as the temperature and the oxygen concentration used in the production.
• the present invention has enabled to provide a ferrite core which has high saturation flux density at 100° C. and 150° C. as well as reduced core loss, and which experiences reduced increase in the core loss in the high temperature storage.
• the ferrite core of the present invention is well adapted for use as a ferrite core in a transformer or a choke coil of a DC—DC converter in an EV (electric vehicle) or HEV (hybrid electric vehicle) which will be used at a high temperature, or as a ferrite core to be placed near an automobile engine which will also be exposed to a high temperature.
• EV electric vehicle
• HEV hybrid electric vehicle

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